This invention relates to a window for use in a supersonic or hypersonic target-tracking missile, the window being transparent to electromagnetic radiation.
Supersonic and hypersonic target-tracking missiles are provided with a seeker head having a seeker responding to electromagnetic radiation. The seeker head detects e.g. the infrared radiation from a target object. When the missile is flying at supersonic or hypersonic speed, the structure of the missile is heated up very strongly due to aerokinetic heating-up. High mechanical and thermal loads occur in the missile and the seeker head. Due to the temperature gradients caused thereby, internal stresses are generated to an extent near the breaking point of the material. Furthermore, extremely high impact pressures act on the structure at high air speeds.
The seeker is protected by a window, which is transparent to the relevant electromagnetic radiation.
There are only very few materials which are transparent to infrared radiation and have mechanical properties sufficient for supersonic and hypersonic speeds. Known electromagnetically transparent windows for supersonic and hypersonic target-tracking missiles are made either of magnesium fluoride, zinc sulfide, sapphire or diamond. Furthermore, depending on the requirement, the windows can have different shapes and thickness. For example, it is tried to increase the thickness of the window in order to counteract the thermal loads by increasing the heat capacity.
Windows made of magnesium fluoride or zinc sulfide have a relatively low thermal conductivity and, thus, they are heated up very much during the flying phase, which lead to melting of the outer surface of the window. Furthermore, the seeker can become "blind" due to the self-emission of the window.
When the windows are made of infrared-transparent materials sensitive to brittle fracture, a larger thickness of the window is favorable for taking up the pressure load. On the other hand, thinner windows have favorable properties with regard to thermal shock loads. There exist an optimum window thickness, for which the stresses of the window caused by impact pressure and temperature are minimal.
All known windows transparent to electromagnetic radiation and used in supersonic and hypersonic target-tracking missiles have the disadvantage that they resist the thermal and mechanical loads during the flying phase just for a short time of 1-2 seconds. Normally, however, the actual time of flight is much longer. Up to now this problem is solved in that the window is protected by a solid protective covering during the first flying phase. Thus, during this flying phase, the seeker is covered and target-tracking is not possible. Not until the final flying phase, when the missile already is close to the target, the protective covering is thrown off, which enables the actual target-tracking by the seeker. Such a protective covering is described in German Patent Application No. 37 15 085. Apart from the limitation of the time of target-tracking, the use of such a protective covering is of rather complex design.
European Patent Application No. 0 599 035 discloses a connecting arrangement for connecting a dome, which covers a seeker head and is made of relatively brittle infrared-transparent material, to the structure of a missile. This connecting arrangement comprises a retaining ring extending over the rim of the dome, positively holding the dome and connected to the structure of the missile. The connection is effected without material interconnection between dome and retaining ring. A groove is provided along the rim of the dome under the retaining ring. A flexible sealing means is provided in this groove for sealing between the retaining ring and the dome.